Download Earthquake`s Seismic Waves

Earthquakes, Earth Interior
Plate Tectonics
Lecture by Dr. Ken Galli, Boston College
EESC116301 Environmental Issues and Resources
July 23, 2015
Please do not distribute beyond the EESC116301 Class.
Earthquakes
• Violent ground-shaking phenomenon caused by the
sudden release of strain energy stored in rocks
• One of the most catastrophic and devastating hazards
• Globally, most earthquakes are concentrated along
plate boundaries
• USGS estimated about 1 million quakes annually
• Millions of people killed and billions of dollars in
damage by catastrophic earthquakes
1/17/1995 Kobe, Japan 5,000 dead; over 300,000 homeless M 7.2, 20 seconds of ground shaking
12/26/2004 Sumatra-Andaman EQ/Tsunami: 283,000 dead/missing, M 9.1, rupture lasted more than 1
hour! rupture length: 1,300km (807mi), longest known; EVERYWHERE on Earth moved at least 1cm (0.4in)!
3/11/2011 Tōhoku earthquake and tsunami M 9.0 Duration: 6 minutes. Depth: 30 km. 15,887 deaths.
Foreshocks 7 Aftershocks 10,982 (as of 7 July 2014)
New Madrid seismic zone in the Mississippi River Embayment, the most
earthquake-prone region in the United States east of the Rocky Mountains.
Locations of recorded earthquakes since 1974 are shown as crosses.(USGS).
Principle Unconsolidated and Semiconsolidated Sand and Gravel Aquifers of
the U.S. Source: USGS Aquifer Basics (National Atlas of the United States)
Miss. Embymnt Cross-section from Arthur and Taylor (1998)
New Madrid Earthquake
2 AM, Dec. 16, 1811 - Feb. 7, 1812
• Series of quakes
• Est. Mag = 7 - 7.5,Lasted one full minute
• New Madrid, Missouri, Louisiana
Territory, population - 800
• In Kentucky, John James Audubon noted:
“the ground weaved like a field of corn
before the breeze!”
• Steamboat Captain noted that, for a
while, the Mississippi River flowed
UPSTREAM!
• 50,000 square miles affected!
• A 25 sq. mile land area was uplifted 20
feet and newly named Tiptonville Dome
was born!
Plate Boundary and Earthquakes
• Most earthquakes are concentrated along plate
boundaries, and nearly all catastrophic earthquakes
are shallow earthquakes
• Divergent plate boundary: Shallow earthquakes
• Transform plate boundary: Shallow to intermediate
earthquakes
• Convergent plate boundary: Wide zone of shallow,
intermediate, and deep earthquakes; 80% of seismic
energy released along the earthquake zone around
the Pacific rim.
C – convergent
margin
D – divergent
margin
D
C
Subduction zone - region where oceanic crust dips beneath the lithosphere and sinks or
is dragged down into the mantle. Ocean crust is destroyed at subduction zones.
Spreading center - region where ocean crust is created from molten rock in the upper
mantle. This crust if split at the ocean ridges (spreading centers) and pushed or pulled
away from the ridges.
Plate Boundary and Earthquakes
Map of global seismicity
Causes for Earthquakes (1)
• Stress and strain
• Stress: A force exerted per unit area within rocks or
other Earth materials
• Strain: Deformation (size, shape, and orientation) of
rock materials caused by stress
• Rock strength: Rock’s ability to stand a magnitude
level of stress before rupture
Causes for Earthquakes (2)
• Earthquake: Sudden release of strain energy caused
by rock rupture (faulting)
• Earthquakes induced by human activities
 Much smaller magnitude
 Reservoir-induced earthquakes
 Deep waste disposal and earthquake
 Nuclear explosions
Earthquake’s Seismic Waves
• Earthquake’s focus and epicenter
• Seismic wave propagation outward from the focus
• P-wave: Compressional waves, travel fastest
through all physical states of media
• S-wave: Shear waves, travel slower than P-wave, but
faster than surface waves, only propagates through
solid materials
• Surface waves: Moving along the Earth’s surface,
travels slowest, but causing most of the damage
Animation: p, s waves
Rupture starts at the focus and propagates up, down, and laterally.
Measuring Seismic Waves (1)
• Seismograph or seismometer
• Amplitude of seismic waves: Amplitude of ground
vibration
• First arrival of seismic waves
 Determine the time of earthquake
 Distance to epicenter from a seismograph based
on the difference in arrival time between P-waves
and S-waves
Material Amplification
• Seismic waves travel differently through
different rock materials
• Propagate faster through dense and solid rocks
• Material amplification: Intensity (amplitude of
vertical movement) of ground shaking more severe
in unconsolidated materials
• Seismic energy attenuated more and propagated less
distance in unconsolidated materials
Generalized relationship between near-surface Earth material and
amplification of shaking during a seismic event.
Earthquake Magnitude Scale (1)
• Richter scale: The amplitude of ground motion
 Increasing one order in magnitude, a tenfold
increase in amplitude
• Moment magnitude scale
 Measuring the amount of strain energy released
 Based on the amount of fault displacement
 Applicable over a wider range of ground
motions than Richter scale
• Earthquake energy: Increase one order in magnitude,
about a 32-times increase in energy
Earthquake Magnitude Scale (2)
Worldwide magnitude and frequency of earthquakes by descriptor
classification. (U.S. Geological Survey. 2000. Earthquakes, facts and statistics.
http://neic.usgs.gov. Accessed 1/3/00)
Earthquake Intensity Scale (1)
• Modified Mercalli Scale
 12 divisions
 Qualitative severity measurement of damages
and ground movement
 Based on ground observations, instead of
instrument measurement
 Scale depending on earthquake’s magnitude,
duration, distance from the epicenter, site
geological conditions, and conditions of
infrastructures (age, building code, etc.)
Modified Mercalli Intensity Map for the 1971 San Fernando Valley,
California, earthquake (M 6.6). (U.S. Geological Survey 1974.
Earthquake Information Bulletin 6[5])
(a) Instrumental Intensity Map and peak ground acceleration
(percent g) for the 1994 Northridge, California, earthquake (M 6.7)
(U.S. Geological Survey, and courtesy of David Wald).
ShakeMap
Effects of Earthquakes (1)
• Primary effects
 Ground shaking, tilting, and ground rupture
 Loss of life and collapse of infrastructure
 Landslides, liquefaction, and tsunamis
• Secondary effects
 Fires, floods, and diseases
Effects of Earthquakes (2)
• Depending upon the frequency of seismic waves
 Body waves (P and S) having higher frequency
than surface waves
 High frequency waves posing more threats on
low structures
 Low frequency waves posing more impact on
tall structures
 High frequency waves attenuated faster over
distance, higher buildings far away from the
epicenter can be damaged
Earthquake Hazards & Risks
• Earthquake risks
 Probabilistic methods for a given magnitude or
intensity
 Earthquake hazard of an area
 Earthquake hazard of a fault segment
• Seismic hazard maps
• Conditional probabilities for future earthquakes
Earthquake Prediction
• Long-term prediction
 Earthquake hazard risk mapping
• Short-term prediction (forecast)
 Frequency and distribution pattern of foreshocks
 Deformation of the ground surface: Tilting,
elevation changes
 Emission of radon gas
 Seismic gap along faults
 Abnormal animal activities
Response to Earthquake Hazards (1)
• Hazard Reduction Programs
 Develop a better understanding of the source
and processes of earthquake
 Determine earthquake risk potential
 Predict effects of earthquakes
 Apply research results
Response to Earthquake Hazards (2)
• Adjustments to earthquake activities
 Site selection for critical facilities
 Structure reinforcement and protection
 Land-use regulation and planning
 Emergency planning and management:
Insurance and relief measures
Earthquake Warning Systems
• Technically feasible: But only about a minute
warning
• Warning system
 Not a prediction tool
 Can create a false alarm
• Better prediction and better warning system?
Perception of the Earthquake Hazard
• Public preparedness for the earthquake potential,
even psychologically
• Pre-earthquake planning
• Post-earthquake emergency response
• Better response, in terms of engineering structural
designs to minimize the hazard risks
Earthquake Processes
• Faults
 Fault types (normal, reverse, thrust,
and strike-slip fault)
 Mapping faults: Surface fault and buried
subsurface fault
 Fault activity (active, potentially active,
and inactive faults)
 Fault-related tectonic creep
 Global plate boundaries, regional and local
faults
2. Stress on a volume of rock = sum of all geologic forces acting on the
volume.
—downward-directed stresses generated by gravity acting on the
volume itself plus the weight of overlying rocks, oceans, etc.
—upward directed stresses generated by having lower density
rocks underneath higher density rocks. Gravity “says” the rocks with
greater density ought to be below the low-density rocks, so the lighter
rocks tend to flow upwards
—sideways directed stresses generated by plate motions, for
example, or by convection in the mantle.
Stresses are forces that try to deform or move the volume of rock.
If the rock is stationary, then the upwards-directed forces exactly
balance the downward-directed forces.
3. Strain is the amount the rock deforms when stressed by geologic
forces.
Solids can exhibit different kinds of behavior in response to
stress:
Brittle—substance shatters when stressed. Ex: Glass
bottle, rocks in crust at low temperatures and pressures
Plastic—substance can be deformed but will not “snap
back” when stress stops. Examples: Flow of glaciers; rocks when hot
Elastic—substance can be deformed but will “snap back”
to its original shape when stress is released. Examples: rubber band;
wood beam; rocks.
So, rocks exhibit all 3 kinds of behavior depending on the
nature of the stress and the environment (Pressure, Temperature)
the rock is in. Rocks have a certain strength, but if Stress
exceeds strength, rocks will break.
Earthquake Cycles
• Faulting and elastic rebound
• Stages of earthquake cycle
 Inactive and aftershock stage
 Stress accumulation stage
 Foreshocks
 Main shock (major earthquake)
• Earthquake cycle over time: Recurrence intervals
• Earthquake cycle in space: Seismic gaps
1. As rock is deformed it bends,
storing elastic energy
2. When rock is strained
beyond its breaking point it
ruptures, releasing the stored
up energy in the form of
earthquake waves.
From time 1 to
time 4 may take
from hundreds to
thousands of
years.
4. Seismicity of an area – EQ Activity. Lots of EQs “High
Seismicity”
5. Fault—plane along which rocks break. Real faults are irregular,
wavy shaped, not smooth cuts like knife through butter. In order for 2
blocks on opposite sides of a fault to move, must build up enough Strain
energy in rocks near fault to overcome friction and/or strength of rock
chunks that “lock” the fault.
Fault (def.) a fracture with movement
—KINDS OF FAULTS EQs generally associated with fault.
For simplicity, suppose rocks either move vertically OR horizontally
only along a fault.
(real faulting usually would have some vertical and some horizontal
motion).
Reverse Fault
Normal Fault
Types of fault movement based on the sense of motion relative to the fault.
Types of fault movement based on the sense of motion relative to the fault.
1. Construct mine tunnel (rectangle or square) – let fault
be a diagonal of the tunnel.
2. Stand inside the tunnel. Your feet are on the lower block,
or the Footwall Block.
3. If you hang a lamp anywhere on the ceiling, it hangs
from the upper block, or the Hanging Wall Block.
B.) Horizontal Motion Along Faults (no up/down). Don’t need to
worry about hanging wall/footwall here.
2 Types here also: left-lateral strike-slip; right-lateral strike-slip faults.
Strike-Slip Faults—are faults where, due to shearing between rocks,
motion along the fault is predominantly horizontal. One special class of
strike-slip faults is Transform Faults that separate two lithospheric
plates.
Types of fault movement based on the sense of motion relative to the fault.
Animations: normal fault; strikeslip fault producing EQs
II. EQ WAVES (2 general categories)
A) Body Waves—can travel through the body of the Earth
B) Surface Waves—created when body waves poke out at Earth’s
surface; can only travel along Earth’s surface (like ocean waves).
A) Body Waves: P-waves & S-waves
1) P-Waves—
“P” for ‘primary’ arrive first, travel fastest through rock; made up of
alternating compressions and dilations; move rock particles back and
forth in same direction that waves are moving.
2) S-Waves—
arrive second, transverse waves that move rock by a shearing motion, at
90° to wave travel.
Liquids have no shear strength so S waves cannot travel through liquids
(outer core). You can pick up a rug and shake it to make an S wave, but
what happens if you try to dip your hands into a lake, grab some water,
and do the same thing?! No dice. If you need to avoid S-waves, live on
a boat.
B) Surface Waves: Love waves & Rayleigh waves
These only shake the outermost part of the Earth, down to about 2km or
less (+/-). Below that depth, there is no deformation. [Analogy:
snorkeling or scuba diving- 10-30’ down you don’t feel any effects of the
Surface waves—same thing.]
A Love wave is a surface
wave having a horizontal
motion that is transverse (or
perpendicular) to the
direction the wave is
traveling.
A Rayleigh wave is a
seismic surface wave
causing the ground to shake
in an elliptical motion, with
no transverse, or
perpendicular, motion.
Source: USGS
So, a typical seismogram would look something like:
(Avg. Crustal P –wave Velocity ~6.2 km/sec)
S-wave velocity is ~60% of P-wave velocity.
The propagation velocity of the waves depends on density and elasticity
of the medium. Velocity tends to increase with depth, and ranges from
approximately 2 to 8 km/s in the Earth's crust up to 13 km/s in the deep
mantle.
Animation: passage of P and S
waves
Reflection—wave passes through rock of 1 density, hits
boundary of rock with suddenly different density part of the wave’s
energy reflects back into the first substance.
Refraction—part of wave’s energy passes through into the
second substance, but wave has changed direction. Ex. Pond
Same thing with body waves reflect/refract when encounter rocks with
different density.
2) Determine structure of whole planet—arrival times of
various waves from very large EQs at stations all over the Earth.
Interpret whole Earth structure by looking at times of arrival of various
kinds of earthquake waves.
next slide--
Inertia of the suspended mass tends to keep it motionless, while
the recording drum, which is anchored in bedrock, vibrates in response to
seismic waves. Thus, the stationary mass provides a reference point
from which to measure the amount of displacement occurring as the
seismic wave passes through the ground below.
Source: www.tulane.edu/~sanelson/geol204/eqcauses.htm
Travel-time graphs are used to tell the distance from the receiving
seismograph and the epicenter of an earthquake. Three or more
seismograph stations are needed to pinpoint the epicenter.
In 1902, Giuseppe Mercalli produced the Mercalli Intensity Scale based on the
amount of damage caused by an EQ.
In 1935, Charles Richter, at CIT, made the Richter Magnitude Scale based on the
magnitude or energy released by an EQ. This is a logarithmic scale; therefore, an increase of one
on the scale represents a ten-fold increase in the amplitude (height) of the wave. Each increase in
1 in Richter Magnitude represents a 32-fold increase in the amount of energy released. Thus, a
magnitude 7 earthquake releases 32 times more energy than a magnitude 6 earthquake. A
magnitude 8 earthquake releases 31 x 31 or 1024 times as much energy as a magnitude 6
earthquake.
The modifiers that the press uses when describing EQs is:
Structural damage caused by EQs can be due to:
1. Intensity and Duration of vibrations
2. Nature of Materials upon which structures rest
3. Design of Structures
Ex. 1964 Anchorage AK EQ – ground shaking was important
1556 China EQ – structures were built on loess (fine silt), which collapses
easily with vibration, and resulted in great loss of life.
Tsunami
Fire
Seismic sea waves; not generated by tidal effect of moon.
Result from vertical displacement of ocean floor during an EQ
Tsunamis move at speeds of 500-950 km/hr [300-600 mph!]
First warning is the rapid withdrawal of water from beaches
Fire can be a very important cause of destruction associated with EQs
Fire was very important in the destruction associated with the 1906 EQ near
San Francisco, CA. Wood buildings and broken gas and electrical lines/pipes
led to incredible devastation.
Landslides and Ground Subsidence
Important in 1964 Anchorage AK EQ, where landslides and ground
subsidence caused the most damage to structures.
Plate Setting: 1/17/1995
7.2 Kobe EQ
20 seconds ground shaking
5,000 dead; >300,000 homeless
2004 Sumatra EQ Overview Map
Rupture Length: 1300 km (807mi) longest of any known EQ
Rupture lasted over an hour-longest ever
No point on Earth was undisturbed, with peak ground motion greater
than 1cm (0.4in) Everywhere!
Energy released was equiv. to total energy used in U.S. in 6 months.
Rupture started at depth of 30km (18.7mi).
Source: http://www.semp.us/publications/biot_reader.php?BiotID=187
Plate Setting: 12/26/2004 Mag 9.1
Sumatra-Andaman EQ/Tsunami: 283,000 dead/missing
THE EARTH’S INTERIOR
Tectonics in a Nutshell, USGS
4. Once formed, convection currents bring hot material from deeper
within the mantle up toward the surface.
5. As they rise and approach the surface, convection currents diverge at
the base of the lithosphere. The diverging currents exert a weak tension
or “pull” on the solid plate above it. Tension and high heat flow weakens
the floating, solid plate, causing it to break apart. The two sides of the
now-split plate then move away from each other, forming a
DIVERGENT PLATE BOUNDARY.
6. The space between these diverging plates is filled with molten rocks
(magma) from below. Contact with seawater cools the magma, which
quickly solidifies, forming new oceanic lithosphere. This continuous
process, operating over millions of years, builds a chain of submarine
volcanoes and rift valleys called a MID-OCEAN RIDGE or an
OCEANIC SPREADING RIDGE.
7. As new molten rock continues to be extruded at the mid-ocean ridge
and added to the oceanic plate (6), the older (earlier formed) part of the
plate moves away from the ridge where it was originally created.
8. As the oceanic plate moves farther and farther away from the active,
hot spreading ridge, it gradually cools down. The colder the plate gets,
the denser (“heavier”) it becomes. Eventually, the edge of the plate that is
farthest from the spreading ridges cools so much that it becomes denser
than the asthenosphere beneath it.
9. As you know, denser materials sink, and that’s exactly what happens
to the oceanic plate—it starts to sink into the asthenosphere! Where one
plate sinks beneath another a subduction zone forms.
10. The sinking lead edge of the oceanic plate actually “pulls” the rest
of the plate behind it—evidence suggests this is the main driving force of
subduction. Geologists are not sure how deep the oceanic plate sinks
before it begins to melt and lose its identity as a rigid slab, but we do
know that it remains solid far beyond depths of 100 km beneath the
Earth’s surface.
11. Subduction zones are one type of CONVERGENT PLATE
BOUNDARY, the type of plate boundary that forms where two plates are
moving toward one another. Notice that although the cool oceanic plate
is sinking, the cool but less dense continental plate floats like a cork on
top of the denser asthenosphere.
12. When the subducting oceanic plate sinks deep below the Earth’s
surface, the great temperature and pressure at depth cause the fluids to
“sweat” from the sinking plate. The fluids sweated out move upward,
helping to locally melt the overlying solid mantle above the subducting
plate to form pockets of liquid rock (magma).
13. The newly generated molten mantle (magma) is less dense than the
surrounding rock, so it rises toward the surface. Most of the magma
cools and solidifies as large bodies of plutonic (intrusive) rocks far below
the Earth’s surface. These large bodies, when later exposed by erosion,
commonly form cores of many great mountain ranges [such as the Sierra
Nevada (California) or the Andes (South America)] that are created along
the subduction zones where the plates converge.
14. Some of the molten rock may reach the Earth’s surface to erupt as
the pent-up gas pressure in the magma is suddenly released, forming
volcanic (extrusive) rocks. Over time, lava and ash erupted each time
magma reaches the surface will accumulate—layer upon layer—to
construct volcanic mountain ranges and plateaus, such as the Cascade
Range and the Columbia River Plateau (Pacific Northwest, U.S.A.).
http://www.uwgb.edu/dutchs/EnvSC102Notes/102HowEarthWorks.HTM
Tsunami Warning System